(1) Field of the Invention
The present invention relates to a removable CMP polishing pad, devices using the same and a method for chemical mechanical polishing.
(2) Description of Related Art
Polishing Pads together with slurry and diamond conditioner disks form the key components of the equipment used to carry out CMP processes in recent years. These pads have been produced and marketed by several vendors to standards of reliable quality and effectiveness. The function of the polishing pad is to provide the base upon which the mechanical action of the slurry on the wafers may be carried out by limiting the space between the pad and the wafer and thereby forcing the abrasive components of the slurry into contact with the wafer surface in a manner that renders regular and suitable mechanical abrasion. For this purpose it has long been known in the art that the surface of the pad should maintain a certain roughness not only for direct mechanical abrasive action at a desirable pace but also to provide certain spacing on the surface of the pad for transport of the slurry. If the surface of the pad becomes too smooth, these objectives are not met and the CMP process is not effective.
To overcome this problem, CMP processes employ a diamond conditioner disk that continually roughens the surface of the polishing pad to counteract the gradual flattening of the pad surface that occurs due to abrasion and plastic deformation from the slurry and wafer. This roughening or conditioning results both in the desired effect of continued effective mechanical abrasion of the wafer and gradual reduction of the polishing pad thickness. The ability of the conditioning disk to maintain roughness is clearly important because if the pad is not rough enough, mechanical abrasion will not occur at the desired pace. Moreover, the wearing away of the pad thickness is important because it eventually leads to the replacement of the pad.
Needless to say different processes have certain desirable degrees of roughness and other surface characteristics and acceptable rates of wear and these can be adjusted by the manner in which the pad is manufactured, the material it is manufactured from and the manner of its use.
One of the problems presently encountered in CMP processing is that to be economically effective the pad needs to be in use for long continuous periods of time. In present practice the pad is attached to the CMP polishing platen using a double-sided adhesive film and CMP processing is done for long periods of time. In general the characteristics of pad wear are known but as slurries and wafer characteristics and the materials that are polished are changed it can be expected that surface characteristics and wear rates can be affected. It is of considerable economic importance to gain an accurate understanding at the start or modification of each CMP process of specifically how the pad is going to wear, how rough it is going to be and how fast it is going to wear.
Presently the polishing pad in either a production or research environment is used for what is considered a suitable length of time (up to several tens of hours) and then stripped from the platen, a process that irreversibly deforms or damages the pad and its adhesive layer and usually precludes further use. A new pad is then installed, broken in with the conditioner, seasoned with slurry, and qualified for polishing product wafers. A pad changeover can require several hours of tool down time and loss of productivity. For some CMP work where conditions change after exhausting a relatively small amount of pad the loss in productivity can be significant. In addition to productivity loss, several kinds of surface measurements that could be useful for monitoring the condition of the pad during break-in and processing must be deferred until the pad is removed, or else a sample must be physically cut from the pad for such monitoring, a practice that often deforms the sample and always damages the pad. It is desirable that there be a process or device making it possible to easily and cost effectively perform measurements on the pad surface without having to irreversibly remove the pad or a sample of the pad, without having to wait for the end of the process and without engendering a significant amount of lost time or use of resources.
To date, no effective method has been disclosed by manufacturers for facilitating the taking of measurements of and exchange and storage of pads during operation and users have had to rely shutting the CMP process down and either removing the pad or a sample of the pad and measuring it, which is neither cost effective from the standpoint of time nor conducive to desired levels of quality control. Removal of the pad or a sample has meant distortion or irreversible damage of the pad surface and the pad making measurements of the pad surface of questionable use and validity and generally rendering the pad unusable thereafter. The alternative is to wait for the end of the process to conduct measurements, in which case wastage may have occurred during the initial run and which in any case will not provide information about wear during the initial stages of use of the pad. Even in this case it is generally true that pad removal will damage and put at risk any measurement data that might have been obtained.
Methods to solve this problem have included, for example, U.S. Pat. No. 6,422,921 which describes a method for detaching the CMP pad by means of the use of an adhesive t. However in the method taught the pad can be softened by heating it to the point where the platen will release the pad so that it can be removed.
However, this method, although it allows the pad to be removed more easily that before, is limited by the fact that considerable additional equipment is needed to make it possible, the fact that temperature and other conditions of CMP must be limited to avoid the risk of the CMP pad coming loose when it is not desired that it be removed and finally, this method does not solve the problem of deformation of the CMP pad surface when it is removed from the platen.
Moreover, U.S. Pat. No. 7,156,722 (filed Oct. 28, 2005) describes a removable platen that is affixed by a vacuum apparatus. However, although this application describes a method and apparatus where the platen can be removed intact, it is not clear that the vacuum supporting detachable part of the platen is easily suitable for introduction into large stage measuring devices, the vacuum must be created, maintained and released which add considerable additional complexity to the operation of this type of method and apparatus, conditions must be more carefully maintained to prevent material from entering areas that would create a problem for reestablishing the vacuum seal, the apparatus will clearly involve significant additional cost and once removed from the CMP apparatus, the pad must still be removed from the detachable portion of the platen before a new pad can be used.
Additionally, the XCEDA tool of Novellus Systems is a detachable upper surface on which the pad rests that can be removed and strictly speaking, allows the advantages of being able to remove a pad for metrology or storage and then later returning it to use for polishing. However, the pad and detachable upper surface are relatively high cost, the means of detachment, while it is not destructive of the pad is not particularly convenient, the pad is opaque which does not allow inspection of the lower side to determine if the pad is well seated with no bubbles or not and the system cannot be retrofitted to fit any tool or polisher.
Users of CMP Polishing Pads need to know that they are receiving the same quality of product from the polishing pad manufacturers from the standpoint of process effectiveness on a consistent basis and such a device or method would allow users to better determine specifications for what they require. Users also want to know, as stated above, how well their pads are faring under certain operating conditions and a method or device that will allow them to an accurately determine pad surface conditions and wear will provide them with useful information in that regard. Finally, from a research and development standpoint, the results of such a test would provide makers of CMP polishing pads with more useful information about how to improve existing manufacturing processes for CMP Polishing pads and provide better technical support to specific users and in the general development of new CMP and related processes.
As observed, above removable pad systems have been known in the prior art but they were not design for the purpose of providing secure and accurate replacement of the pad on the platen once it has been removed. The inventor of the present invention was unable to find in the prior art any method or device to suitably overcome this problem. More particularly, the inventor of the present invention was motivated by the fact that when pad samples were removed for optical interferometry, they were often visibly distorted after being cut and pried from the pad. This distortion had to be modeled and removed mathematically from the data. When subtle surface changes were likely to be present, samples cut and analyzed in this way were often of questionable value. The problem of how to accomplish a removable and replaceable pad for regular use on the CMP platen in a way that would maximize the ease and speed of removal, the integrity of the pad surface anticipating further use of the pad in CMP and the secure and essentially immobile condition of the two sheets with respect to each other, the platen and the pad once they were secured together were the object of sustained and focused investigation by the inventor of the present invention.
The present invention seeks to provide a useful device and an accurate and consistent method for allowing the simple, time saving and cost effective removal of CMP pads permitting the easy measurement and determination of surface characteristics of the CMP pad during use, exchange with other CMP pads and storage of used CMP pads with remaining useful life. These and other advantages of the invention will be apparent from the description of the invention provided herein.